‘Tachykinin’ is a collective term for such neuropeptides as substance P, neurokinin A, and neurokinin B. These tachykinins are known to exhibit various physiological activities by binding to corresponding receptors in a human body (neurokinin 1 (NK1), neurokinin 2 (NK2), and neurokinin 3 (NK3), respectively). Of different tachykinins, substance P, aside from its role as a neurotransmitter in primary sensory neurons in central and peripheral nervous systems, brings about various physiological effects, such as diuresis, excitation of neurons, increased blood vessel permeability, blood vessel dilation, contraction of smooth muscles, and immune activities. Substance P is also believed to play significant roles in the onset of various pathological conditions such as pollakiuria, incontinence, vomiting, inflammation, allergies, respiratory tract disorders, pains, and central nervous system disorders. Thus, a need exists for the development of a compound that acts as a tachykinin receptor antagonist and, in particular, as an NK1 receptor antagonist and is thus suitable for use as an effective prophylactic or therapeutic agent against various pathological conditions such as those mentioned above. It is also desirable that such a compound offer high safety, persistence of efficacy, and other advantageous characteristics.
At present, the following compounds are known as NK1 receptor antagonists and are described in the following publications:                (1) European Patent Application Publication No. EP-A-429366 describes compounds such as the one represented by the following formula:         (2) PCT pamphlet (International Patent Publication) No. WO91/09844 describes compounds such as the one represented by the following formula:         (3) European Patent Application Publication No. EP-A-532456 describes compounds such as the one represented by the following formula:         (4) European Patent Application Publication No. EP-A-522808 describes compounds such as the one represented by the following formula:         (5) PCT pamphlet (International Patent Publication) No. WO93/01169 describes compounds such as the one represented by the following formula:         (6) Japanese Patent Laid-Open Publication No. Hei 8-67678 describes a compound represented by the following formula and salts thereof: wherein the rings A and B are each a homocyclic or heterocyclic ring with at least one of the rings A and B being a heterocyclic ring; the ring C is a benzene ring; R is H or a hydrocarbon residue; one of X and Y is —NR1— (where R1 is H or a hydrocarbon residue) or —O— and the other is —CO— or —CS—, or one of X and Y is —N═ and the other is ═CR2— (where R2 is H, a halogen, a hydrocarbon residue, an amino, or hydroxyl group); and n is 1 or 2.        (7) Japanese Patent Laid-Open Publication No. Hei 9-104674 describes a compound represented by the following formula: wherein X is a hydrogen or oxygen atom; Y is a nitrogen or oxygen atom which may or may not be alkylated or acylated; R1 is a hydrogen atom, a lower alkyl group, a lower alkanoyl group, an alkyl group having a nitrogen atom, a carbamoyl group, a lower alkylthio group, a lower alkylsulfinyl group, a lower alkylsulfonyl group, or a (4-phenylpiperadine-1-yl)methyl group; R2 is a hydrogen atom, a lower alkyl group, a lower alkyl group having a hydroxyl group, a lower alkanoyl group, or a lower alkoxy group; and the rings A and B are each a substituted or unsubstituted benzene ring.        (8) Japanese Patent Laid-Open Publication No. Hei 9-263585 describes a compound represented by the following formula: wherein the ring M is a heterocyclic ring in which the structural moiety —X═Y< is —N═C<, —CO—N<, or —CS—N<; Ra and Rb may together form the ring A, or Ra and Rb are each independently a hydrogen atom or a substituent of the ring M; the rings A and B are each independently a substituted or unsubstituted homocyclic or heterocyclic ring, provided that at least one of the rings A and B is a substituted or unsubstituted heterocyclic ring; the ring C is a substituted or unsubstituted homocyclic or heterocyclic ring; the ring Z is a substituted or unsubstituted ring; and n is an integer from 1 to 6.        (9) Japanese Patent Laid-Open Publication No. Hei 11-246559 describes a compound represented by the following formula: wherein X is a nitrogen atom or a CH group; R1 is a hydrogen atom, a lower alkyl group, an aryl group, or an aralkyl group; R2 is a hydrogen atom or a lower alkyl group; the rings A and B are each independently a substituted or unsubstituted benzene ring; and n is 1 or 2.        (10) Japanese Patent Laid-Open Publication No. 2000-139834 describes a compound represented by the following formula: wherein R1 and R2 are each independently a hydrogen atom or an C1 to C6 alkyl group; R3 is a hydrogen atom, a substituted or unsubstituted C1 to C6 alkylcarbonyl group, a substituted or unsubstituted C1 to C6 alkylsulfonyl group, a substituted or unsubstituted C1 to C6 alkyl group, a substituted or unsubstituted arylmethyl group or an alkoxycarbonyl group; the ring A is a homocyclic or heterocyclic ring which may include 1 through 3 independently selected substituents (any adjacent two of which may be bound to one another to form a ring); the ring B is a benzene ring which may include 1 through 5 substituents (any adjacent two of which may be bound to one another to form a ring); and the ring C is a benzene ring which may include 1 through 3 substituents (any adjacent two of which may be bound to one another to form a ring).        (11) Japanese Patent Laid-Open Publication No. 2000-247957 describes a compound represented by the following formula: wherein R is a hydrogen atom or the like; R1 is a hydrogen atom or the like; R2 and R2′ are each a hydrogen atom or the like; R3 is a hydrogen atom or the like; R4 is a hydrogen atom or the like; R5 is a hydrogen atom or the like; R6 is a hydrogen atom or the like; X is —C(O)N(R5)— or the like; n is an integer from 0 to 4; and m is 1 or 2.        (12) PCT pamphlet (International Patent Publication) No. WO00/50401 describes a compound represented by the following formula: wherein R is a hydrogen atom or the like; R1 is a hydrogen atom or the like; R2 is a hydrogen atom or the like; R3 is a hydrogen atom or the like; R4 is a hydrogen atom or the like; R5 is a hydrogen atom or the like; R6 is a hydrogen atom or the like; X is —C(O)N(R5)— or the like; n is an integer from 0 to 4; and m is 1 or 2.        (13) PCT pamphlet (International Patent Publication) No. WO00/73279 describes a compound represented by the following formula: wherein R1 is a hydrogen atom or the like; R2 is a hydrogen atom or the like; R3 is a hydrogen atom or the like; R4 and R4′ are each a hydrogen atom or the like; R5 is a lower alkyl group or the like; n is an integer from 0 to 2; and X is —C(O)N(R4″)— or the like.        (14) PCT pamphlet (International Patent Publication) No. WO00/73278 describes a compound represented by the following formula: wherein R1 is a hydrogen atom or the like; R2 is a hydrogen atom or the like; R3 is a hydrogen atom or the like; R4 and R4′ are each a hydrogen atom or the like; R5 is a lower alkyl group or the like; R6 is a hydrogen atom or the like; n is an integer from 0 to 2; and X is —C(O)N(R4″)— or the like.        